Latch unit and assembly, and method of operating a latch unit

ABSTRACT

The invention relates to a latch unit which includes a bolt, a housing for the bolt, guides for the bolt in the housing, a first spring for the bolt in the housing, the first spring urging the bolt in a forwards direction towards a holding position in which an end part of the bolt is outside the housing, and a slot arrangement for the bolt, the slot arrangement comprising a first part and a second part, the slot arrangement having an active condition in which the first part is engaged with the second part whereby to retain the bolt against movement in the said forwards direction, characterized by a second spring to hold the first and second parts in the active condition. It is a particular feature of the invention that the bolt has a holding position in which it can engage a keeper, a non-holding position in which it is disengaged from the keeper and a retained position in which it cannot engage the keeper and in which it is retained by the slot arrangement, the non-holding position being between the holding and retained positions. There is also provided a method of operating a latch unit according as herein defined.

FIELD OF THE INVENTION

This invention relates to a latch unit and assembly, and to a method ofoperating a latch unit. In particular it relates to a latch unit havinga slidable bolt, typically for securing a movable panel to a fixedmember, such as to a part of a frame for the panel.

In this specification, “left” and “right”, “forward”, “rearward” andsimilar geometric terms refer to parts in a typical orientation of useas shown in FIG. 1, unless otherwise specified.

BACKGROUND TO THE INVENTION

When it is desired to secure a movable panel to a fixed member such asto an upright of an outer frame member, it is known to use a slidingbolt mounted to the panel, the bolt end being fittable behind or withina keeper carried by the frame, whereby to effect the securement. Thekeeper typically is a plate mounted over a recessed portion of the fixedmember or an apertured body secured to the fixed member. The paneltypically is a hinged panel which closes perpendicularly or nearly so tothe fixed member and thus to the keeper. In use the bolt is slidablebetween (a) a holding or forwardly extended position in which the boltend can fit within the recess or aperture provided by the keeper wherebyto secure the panel in the closed condition, and (b) a rearwardlyretracted or non-holding position in which the bolt end cannot engagethe keeper whereby to allow the panel to move relative to the fixedmember.

Usually the bolt is mounted in a bolt housing, and is slidable betweenguides carried by the bolt housing. Various types of bolt and boltassembly are known, some of which are lockable in the holding position.

One type of bolt assembly has resilient bias means for the bolt, theresilient bias means conveniently being a compression spring or a leafspring mounted in the bolt housing, the spring urging the bolt endtowards the holding position, the bolt end having a chamfer; in thecondition of use the chamfer faces the keeper as the panel is beingclosed. As the panel is moved to its closed condition the keeper forcesback the bolt against the spring and into its non-holding position,until the bolt end is aligned with the recess or aperture whereupon thespring urges the bolt into its holding position; this action is oftenreferred to as latching, and the spring as a latch spring, whilst thebolt assemblies of which the biassed and chamfer-ended bolts form apart, are referred to herein as latch units.

One common form of latch unit is designed to be fitted to the interiorsurface at the rim of an inwardly opening door, such as a front door ofa domestic dwelling. For simplicity, the following disclosure will referto a latch unit for such use (rim latch); however, the latch unit of theinvention can also be morticed into a door or other panel, and can befitted to windows and other movable panels. The keeper for such latchunit will usually be formed with an aperture, sized to receive the boltend and needing only to be affixed to the door frame or other fixedmember in order to provide in the holding position a self-containedembracement of the bolt end, and for simplicity also the followingdisclosure will assume such a keeper.

A latch unit typically has a bolt which can be moved rearwardly(retracted) by an actuator. The actuator will often be connected at oneside to a “thumb-turn” (usually the side which will be to the inside ofthe building in use), the thumb-turn being rotated to move the bolt fromits holding to its non-holding position. At its other side the actuatorwill often be connected to the barrel of a lock (usually the side whichwill be to the outside of the building in use), so that from that sideit is necessary to insert a key to retract the bolt from its holdingposition.

Because the bolt of a latch unit would otherwise automatically enter thekeeper (“latching” action) when the door is closed, it is common torequire a device to be fitted which an occupier can operate in order topermit the force of the latch spring to be over-ridden. Such device willtherefore permit the bolt to be retained in its non-holding positioni.e. non-latching condition, so that the occupier for instance does notneed to use a key or other operating means in order to regain entry intothe premises. A means to over-ride the latch spring, and to maintain thebolt away from its holding position is often referred to as a“latch-back” device.

DESCRIPTION OF THE PRIOR ART

It is known to provide a latch-back device for a rim latch, the devicetypically comprising a slidable plate which can be moved substantiallyperpendicularly to the direction of movement of an extension secured toa short latch bolt. Whilst the latch bolt is being held in itsnon-holding position, as by a thumb-turn, the plate can be moved intoengagement with a recess or the like in the extension for the latch boltso that thereafter the plate retains the extension and thus the latchbolt in its non-holding position. When the occupier or other persondesires to re-activate the latch unit, the plate is moved to release theextension whereby to permit the latch bolt to move axially, specificallyto be urged to its holding position by the latch spring.

It is possible to operate this known latch-back device with one hand,but this is difficult and often awkward to achieve, so that in practicesetting the latch-back device often requires the simultaneous use ofboth hands, one hand to rotate and hold the thumb-turn and the otherthen to move the plate. In addition, the operating means for the plateis small, and can be difficult to grip so that the latch-back device maybe seldom or never used by the elderly or disabled. It is believed thatthese known (two-handed) latch-back devices are particularly difficultto operate for those suffering from arthritis or rheumatism. The knownlatch back devices are not safe to use on the safety exit (“panic”)doors of multi-occupier premises.

STATEMENT OF THE INVENTION

We seek to provide a latch unit having a retaining means or latch-backdevice which reduces or avoids the disadvantages with the known devices,and which can be simpler to operate. In particular, we seek to provide alatch unit in which the latch-back device can automatically re-set, andwhich can thereafter be released by one hand. We therefore seek toprovide a latch unit in which the latch-back device does not require alatch-back plate or the like to be moved by the user whilst the bolt issimultaneously being held manually in a retracted position against theforce of the latch spring.

According to one feature of the invention we provide a method ofoperating a latch unit, the latch unit having a bolt, the bolt beingmovable in a first direction between a holding position in which a partof the bolt projects from the housing and a non-holding position inwhich the said part projects less from the housing, the bolt beingbiassed towards the holding position, the latch unit having actuatingmeans for moving the bolt in said first direction from its holdingposition to its non-holding position against said bias, the bolt havinga retaining means to retain it at a position away from its holdingposition, characterised in that the retaining means can be activated toretain the bolt only upon further movement of the bolt in the said firstdirection to a position rearwards of the non-holding position.

The retaining means is automatically activated upon said furthermovement of the bolt in the said first direction. The retaining means isconveniently a pivoted lever, or alternatively a slidable plate orlever.

Preferably, in the non-holding position the said part does not projectfrom the housing, perhaps being flush with a planar outer forwardsurface of the housing.

It is thus a feature of this invention that the bolt may be moved by theactuating means in the first direction from its holding position to itsnon-holding position and that only upon further or continued movement insaid first direction effected by the actuating means can the retainingmeans be effective. In particular, we provide a method of operating alatch assembly which includes a latch unit secured to a movable paneland a keeper secured to a fixed member, the latch unit including a bolthaving a bolt end, the bolt being urged by a first resilient bias meansin a forward direction such that in a closed condition of the assemblythe bolt end is located by the keeper in a holding condition whereinopening movement of the panel relative to the fixed member is inhibited,the bolt end being shaped so that during relative movement of the paneland fixed member towards the said closed condition of the assembly thebolt end can be moved by the keeper against the first resilient biasmeans rearwardly into a non-holding position in which said openingmovement of the panel relative to the fixed member is not inhibited bythe keeper, there being actuating means alternatively to move the boltend into said non-holding position, and retaining means engageable withthe bolt to hold the bolt against forward movement characterised byengaging the retaining means with the bolt only when said bolt isrearward of said non-holding position.

It is a feature of the invention that the retaining means is engagedwith the bolt only when the bolt has been moved positively rearwards ofsaid non-holding position, for instance direct movement as by afinger-pull from the non-holding position or indirect movement as byoperation of a “panic bar” (with the bolt not pausing at the non-holdingposition). The retaining means is not used to secure the bolt in thenon-holding position, but only in the retained position rearwardlythereof.

The bolt is slidably mounted in a housing, the housing carrying a secondresilient bias means, the second resilient bias means urging theretaining means towards the bolt and into a bolt retaining condition.

Thus with the bolt end in its first holding position being held by akeeper, the bolt may then be moved from that holding position to itsnon-holding position (as by a thumb-turn), but the (retracted) boltposition reached following this movement of the bolt is not sufficienteither to activate the retaining means or to permit the retaining meansto be activated i.e. a further movement of the bolt by means of afurther (partial) rotation of e.g. the thumb-turn, is required. In analternative embodiment suited to operation by a panic button or bar, thebolt can be moved directly to the retained position i.e. through andbeyond the non-holding position suited for normal panel (typically anexit door for a multi-occupier building) opening and closing.

The bolt may simply be a latching bolt but preferably is securable inthe holding position to provide a combination latch bolt and lockingbolt.

According to another feature of the invention, we provide a latch unitwhich includes a bolt, a housing for the bolt, guides for the bolt inthe housing, first resilient bias means for the bolt in the housing, thefirst resilient bias means urging one end of the bolt in a forwardsdirection towards a holding position externally of the housing, andretaining means for the bolt, the retaining means having an inactiveposition in which the bolt is free to move relative to its guides and anactive position in which the retaining means provides a holding actionwhereby to retain the bolt against movement in the said direction,characterised in that the retaining means is resiliently biassed towardsits active condition.

According to a further feature on the invention, we provide a latch unitwhich includes a bolt, a housing for the bolt, guides for the bolt inthe housing, first resilient bias means for the bolt in the housing, thefirst resilient bias means urging one end of the bolt in a forwardsdirection towards a holding position externally of the housing, andretaining means to provide a holding action whereby to retain the boltagainst movement in the said direction, characterised in that theretaining means comprises a peg and slot arrangement, and in that asecond resilient bias means effects engagement between the peg and slotto permit said holding action.

The peg and slot are arranged such that the retaining means holds thesaid one end of the bolt rearwards of the non-holding position reachedby the bolt end when pushed back by a keeper during latching i.e. in thelatch back position.

Preferably, the slot is carried by a lever. The second resilient biasmeans acts against one of the said peg and lever. A disengaging meanscan be urged against said one of said peg and slotted lever to overcomesaid second resilient bias means whereby to permit the bolt to be urgedtowards the holding position by the first resilient bias means.

Preferably, the disengaging means is an extension of the lever, thedisengaging means projecting externally of the housing. Preferably theslot is in a movable lever and the peg is carried by the bolt.Desirably, the slot is connected to a second slot, the peg being movablealong said second slot by said first resilient bias means when saidsecond resilient bias means has been overcome by the disengaging meanswhereby the bolt end is moved into said holding position by the firstresilient bias means.

Usefully, the second slot has a ramp portion; usefully the firstresilient bias means also holds said peg and ramp portion in engagement.

We also provide a latch assembly which includes a movable panel and afixed member, the panel having mounted thereto a latch unit as hereindefined and the fixed member having mounted thereto a keeper for saidone end of the bolt, the one end being locatable by said keeper wherebyto secure the panel and the fixed member against relative movement, theone end being retainable by the retaining means at a position away fromthe keeper and away from the non-holding position whereby to permitrelative movement between the panel and the fixed member.

Accordingly, when the assembly is correctly fitted, the bolt has threepositions, a (forward) holding position in which it can engage thekeeper, a (rearward) non-holding position in which it cannot engage thekeeper, and a (more rearward) retained “latch back” position in which itcannot engage the keeper and in which it is retained by the retainingmeans. When used to hold open an emergency exit door when operated by apanic control, the bolt may be passed through the non-holding positionwithout dwelling i.e. without the bolt having a stationary intermediateposition.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described, by way of example, with referenceto the accompanying drawings, in which:

FIG. 1 is a side view of a latch unit according to the invention, partlyin section, the bolt being in its holding position;

FIG. 2 is a side view of the latch assembly including the unit of FIG.1, the bolt being in the retained position;

FIG. 3 is a side view of part of an alternative latch unit, the boltbeing in the non-holding position;

FIG. 4 is a side view of another alternative latch unit;

FIG. 5 is a perspective view of a latch assembly including a furtheralternative embodiment of latch unit;

FIG. 6 is a sectional view of part of the latch unit of FIG. 5;

FIG. 7 is another sectional view of the latch unit of FIG. 5; and

FIG. 8 is a side view of part of yet another alternative embodiment oflatch unit.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The latch unit 10 of FIG. 1 comprises a rigid, preferably metallic,housing 12, which carries guides 14 for an axially-extending rigid(preferably metal) bolt 16. In this embodiment the guides 14 comprise anumber of spaced posts upstanding from housing base 18, but inalternative embodiments the guides could be parallel upstanding rails,or a combination of posts and rails.

Bolt 16 has a holding position (FIG. 1), and intermediate non-holdingposition (similar to that of FIG. 3), and a retained position (FIG. 2).

The bolt 16 is biassed leftwards by first or latch spring 20, as shownin FIG. 1 into its holding position in which the bolt end 22 extendsfrom the housing 12, through housing bolt opening 24, sufficient toengage in or behind a keeper. The latch spring 20 is coiled aroundupstanding post 26 and one end acts against post 28. In an alternativeembodiment the latch spring is a coil spring as in the embodiments ofFIGS. 4 and 7.

The latch unit 10 includes an actuator 30 for the bolt, the actuatorhaving an arm 32 which is engageable with a (rear) surface 34 of arecess in the bolt such that clockwise rotation (as viewed) of theactuator causes rearward axial retraction movement of the bolt 16 in afirst direction against latch spring 20; anti-clockwise rotation ofactuator 30 subsequently allows the latch spring 20 to effect forwardmovement of the bolt 16 to the holding position. In a typical rim latchsecured to the inner face of an external door of a building, one side ofthe actuator will be connected to the barrel of a lock, so that theactuator can only be key-rotated from that side—usually the side whichwill face outwardly of the building; the other side (inside thebuilding) of the actuator will typically be connected to a thumb-turn orthe like. Neither the lock barrel nor thumb-turn are shown in thedrawings, since suitable embodiments are well known to and widely usedby those skilled in this art.

In use as a latch assembly 40 as shown in FIG. 2, housing 12 is fixed toa door 42 whilst keeper 44 is fixed to a frame member 46 for the door.The bolt 16 is movable by arm 32 from its holding position as shown inFIG. 1 rightwards to its non-holding position in which the bolt end 22can pass the keeper edge 48, so that the door 42 can be opened orclosed.

It will be understood that one face of the bolt end 22 is chamfered, inknown fashion. In the embodiment shown in FIGS. 1 and 2 the latch unitis designed to be held in the closed condition following latch unitmovement in the direction into the paper, so that it is the unseen faceof the bolt end 22 which is chamfered. It will also be understood thatwhen the latch unit 10 is moved towards its “closed” condition againstkeeper 44, the bolt end 22 is first forced back to its non-holdingposition against the force of latch spring 20 before returning forwardlyto its holding position when bolt end 22 becomes aligned with the keeperaperture. The keeper has a ramp 50 to cooperate with the chamfer of thebolt end 22, to assist the rearwards movement before the forwardslatching action.

In accordance with one aspect of the invention the latch unit 10 alsoincludes a lever 52 which pivots about fixed housing post 54. The bolt16 carries a peg 56. The lever 52 is biassed (anti-clockwise) into theposition shown in FIGS. 1 and 2 by second or retainer spring 58. Thelever 52 has a first slot 60 which is connected to a second slot 62,both slots 60,62 being sized to accommodate the peg 56.

In the holding position of FIG. 1, the peg 56 is adjacent the forwardedge of the second slot 62 (the peg 56 and the forward edge of secondslot 62 thus together determining the holding position of the bolt);rearward (i.e. rightward as drawn) axial retraction movement of the bolt16 between guides 14 (as by clockwise rotation of actuator 30 orengagement of bolt end 22 with keeper 44) causes relative movementbetween the peg 56 and second slot 62.

During the initial relative movement between the peg 56 and second slot62 the peg 56 is moved along slot ramp surface 64 causing the lever 52to pivot clockwise, extending and tensioning the retainer (coil) spring58 and holding peg 56 and ramp surface 64 in engagement; in analternative embodiment the spring 58 could be a leaf spring orequivalent e.g. a torsion spring. This initial movement continues untilthe bolt end 22 reaches the non-holding position, and thus willencompass the normal movement range of the bolt during its latchingaction.

In accordance with a feature of the invention, the actuator 30 can berotated (clockwise as viewed) to effect further axial movement of thebolt 16 in the rearward direction, beyond the non-holding position. Thepeg 56 is thereby moved into alignment with lever first slot 60, withretainer spring 58 then acting to pivot lever 52 anti-clockwise wherebyto locate peg 56 in first slot 60; with the peg 56 so located the bolt16 is held in its retained or latch-back position.

Ramp surface 64 is longer than the distance moved by the bolt endbetween the non-holding position and the holding position, i.e. furtherrearward movement of bolt 22 is required from the non-holding positionto reach the retained or bolt latch-back position.

It will thus be understood that when the bolt 16 is moved to itsnon-holding position (in which the bolt end 22 can pass the keeper edge48, i.e. the bolt end 22 is flush or substantially flush with thehousing face 66), the peg 56 is (only) part-way up the ramp portion 64,and that such rearward retraction movement of the bolt to itsnon-holding position can be by way of either the actuator or by the bolt16 engaging its keeper 44 during door closure. It will also beunderstood that further movement of the bolt 16 is necessary before theretaining means is activated, i.e. before the peg 56 has completed itsmovement up ramp portion 64 and entered and lodged in first slot 60whereby to provide the latch-back condition. Thus, in the retainedlatch-back condition of the bolt shown in FIG. 2, the bolt end 22 iswithin the housing 12, in a position into which it can only be drivenrearwardly by the actuator 30, not by the keeper 44.

As a further feature of the invention lever 52 has an extension 70. Inthis embodiment (a) extension 70 is generally parallel to first slot 60,(b) second coil spring 58 is connected to extension 70, and (c) housing12 has an opening 72 through which a terminal portion of extension 70projects so as to form a “finger-press”. Thus in use, the latch-back canbe released or de-activated by (downwards) finger pressure on extension70, to pivot lever 52 until peg 56 is aligned with second slot 62whereupon latch spring 20 can move bolt 22 forwards either until surface34 of the bolt recess (FIG. 2) abuts arm 32, or (if actuator 30 hasalready been rotated fully anti-clockwise, or is free to be drivenanti-clockwise by the bias of latch spring 20) until the peg 56 engagesthe forward end of the second slot 62.

In an alternative embodiment the extension 70 is connected to a hoop byway of a cord or chain, the hoop being operable to move the lever 52 inthe direction to de-activate the latch-back device, useful for someoneunable to apply sufficient single-finger pressure to extension 70 butwho can apply the weight of their arm. In addition, a rod may beconnected directly to the bolt 16, which rod extends through the rearwall of the housing; the rod can be shaped so as to be more easilygrasped and pulled than the grasping and rotating of a thumb turn, sofacilitating the use of the latch unit especially by the elderly orinfirm.

Although we prefer that the peg be carried by the bolt and that theslots are within the lever, we do envisage that the slots could bereplaced by depressions and that the depressions could alternatively bein the bolt (with the peg on the lever).

In one embodiment the peg 56 stands slightly proud of the lever 52 andprovides a column upon which a guide roof for the bolt rests, so thatthe bolt is in part being guided between the housing base 18 and thisguide roof, as well as between upstanding posts 14; in an alternativeembodiment a removable cover (similar to cover 76 of FIG. 5) for thebolt housing provides the guide roof.

In the alternative embodiment of FIG. 3, the second slot 162 has aramped portion 164 and a forward portion 178 which is substantiallyparallel to the longitudinal axis of bolt 116. Whilst the bolt 116 movesbetween its holding position and its non-holding position as shown, thepeg 156 moves along the forward portion 178. The resistance to suchmovement is provided only by the latch spring 120. When it is desired tomove the bolt to its retained position, further rearwards movement bythe actuator 130 causes the peg to move along ramped portion 164(pivoting lever 152) and into first slot 160 as described in relation toFIGS. 1 and 2. Resistance to such further movement is provided both bythe latch spring 120 and also by the retainer spring (not shown in thisfigure). It can be arranged that there is a noticeable or significantincrease in resistance when it is desired to move the bolt 116 into itsretained position, so that it is less likely that the bolt will befurther moved to its retained position when this is not desired.Alternatively stated, with the embodiment of FIG. 3 it will be apparentto the user (from the sudden increase in the retraction force which hasto be applied to the bolt) when the bolt is being moved from itsnon-holding poisition and towards its retained position (in whichposition the door is openable freely by the occupier and intrudersalike), i.e. so that the retained position is less likely to be adoptedinadvertently.

This embodiment also permits the retainer spring (not shown in FIG. 3)to be untensioned or under minimum tension during the latching action,(i.e. as compared to the embodiment of FIGS. 1 and 2 it is onlynecessary to tension the retainer spring of the FIG. 3 embodiment whilstthe bolt is being “further” moved to its retained position); this willlikely result in an extended life for the retainer spring. In addition,this embodiment minimises wear and frictional resistance between the peg156 and the ramp portion 164 and also between the lever 152 and pivotpost 154, over the traverse of the bolt 116 during its latching actioni.e. between the holding and non-holding positions. Furthermore, duringlatching movement the extension (not shown in FIG. 3, but similar to theextension 70 of FIG. 1) is not moved e.g. inwardly of the housing, butin contrast during further movement of the latch bolt 116 in this firstdirection towards the latch-back position the extension is moved inwardsrelative to the housing noticeably (by the ramped portion 164) beforesnapping out again when the peg 156 enters the first slot 160, so that avisual indication of the activation of the retaining means is alsoprovided.

The arrangement of FIG. 4 is somewhat similar to that of FIG. 3, in thatthe second slot 262 has a ramped portion 264 and also a forward portion278 which is generally parallel to the longitudinal axis of bolt 216.However, the latch unit 210 shown in FIG. 4 is adapted to cooperate withan open ended keeper (not shown) which can enter aperture 280 betweenthe housing 212 and a receptor 282. Bolt 216 has a first holdingposition (as shown), and a second holding position in which the bolt end222 enters receptor 282. The receptor 282 acts as a second keeper, andis carried by the housing 112—as generally described in our copendinginternational patent application PCT/GB96/01530, the disclosure of whichis incorporated herein by reference.

The advantageous embodiment of FIGS. 5-7 shows a latch unit for use as a“panic” bolt e.g. for use on an emergency exit door, in which a button384 can be pressed to move the bolt 316 rearwardly from its holdingposition directly to its retained position (by way of and past itsnon-holding position). In the orientation of FIGS. 5 and 7, rearwardmovement of the bolt is towards the right as drawn, whilst in theorientation of FIG. 6, rearward movement of the bolt is towards the leftas drawn.

In this embodiment the button 384 is mounted on a rod 386 which ispivotably attached to a pivot plate 388. Pivot plate 388 is mounted uponfixed pivot 390, and has an end 392 which engages abutment surface 394of the bolt 316. A particular advantage of this arrangement is that auser of the panic bolt can (and in a panic usually will) move the bolt316 in one movement to the latch-back position, ensuring that the doorremains unlatched and so openable by rescuers from outside. Thiscontrasts with the disadvantage of prior art panic bolts whichautomatically re-latch if allowed to close (so that a person stillinside the building who has been overcome by fumes or who is otherwiseunable themselves to operate the panic bolt to re-open the door isisolated from rescuers outside the building, and can thus be trappedinside). For subsequent normal use, the bolt can be re-set quickly andsimply to the normal latching condition by depression of the fingerpress 370 or equivalent.

It will be understood that the button 384 can be replaced by a pivotingplate, sometimes referred to as a “paddle”, or equivalent operatingmember.

In the embodiment of FIG. 8, the retaining means is slidable, andcomprises a plate 452 slidable between guides 454. The bolt 416 has arecess 460 which can accommodate a peg 456 carried by the retainingplate 452. Retaining plate 452 is biassed upwardly as drawn by coilspring 458. As the bolt 416 is moved between its holding and non-holdingpositions, the peg 456 runs along the bottom edge 462 of the bolt, withthe spring 458 under compression; when the bolt is moved back to itsretained position as shown, the retainer spring 458 urges the peg 456into the recess 460. Thereafter, depression of the plate extension 470(as by finger pressure) can release the peg 456 from the recess,permitting the latch spring 420 to urge the bolt 416 towards its holdingposition.

In this embodiment, the retaining plate 452 lies between the bolt 416and the base 418 of the housing 412, so that the bolt 416 is spaced fromthe base 418 by the thickness of the retaining plate 452 and thecorresponding depth of the guides 454; in an alternative embodiment theretaining plate lies between the bolt 416 and the cover (not shown).

An advantage of the constructions described is that the bolt16,116,216,316,416 can be of substantially constant dimensionsthroughout its length, and its part within the latch housing need not(as in some known latch unit constructions) be of reduced dimensions(and thus of reduced strength against attempted unlawful entry). Thereduced dimensions of these prior art latch-back retaining members maybe necessary so that they can be accommodated within their respectivehousings.

What is claimed is:
 1. A latch unit which includes a bolt, a housing forthe bolt, guide means for the bolt in the housing, first resilient biasmeans for the bolt in the housing, the first resilient bias means urgingthe bolt in a forwards direction towards a holding position in which anend part of the bolt is outside the housing, and retaining means forretaining the bolt in a retained position against movement in saidforwards direction, the retained position being spaced from the holdingposition, the retaining means comprising a first part and a second part,the retaining means having an active condition in which the first partis engaged with the second part whereby to retain the bolt, secondresilient bias means to hold the first and second parts in the activecondition, characterized in that the second resilient bias means acts toresist movement of the bolt to the retained position.
 2. A latch unitaccording to claim 1 characterized in that one of the first part andsecond part of the retaining means is carried by one of: a pivotedlever; a slidable plate; and a slidable lever.
 3. A latch unit accordingto claim 1 characterized in that the first part is a peg and the secondpart is a slot, whereby the retaining means comprises a peg and slotarrangement.
 4. A latch unit according to claim 1 characterized in thatthe bolt can be secured in the holding position whereby to provide acombination latch bolt and locking bolt.
 5. A latch assembly whichincludes a moveable panel and a fixed member, the panel having mountedthereto a latch unit according to claim 1 and the fixed member havingmounted thereto a keeper for said end part of the bolt, the bolt having{i} a holding position in which it can engage the keeper when the panelis in a closed condition with the end part and keeper in alignment {ii}a non-holding position in which it is disengaged from the keeper, and{iii} a retained position in which it cannot engage the keeper and inwhich it is retained by the retaining means, the non-holding positionbeing between the holding and retained positions.
 6. A latch assemblyaccording to claim 5 characterized in that in the non-holding positionsaid end part is substantially flush with an outer surface of thehousing, and in that in the retained position said end part is withinthe housing.
 7. A latch unit according to claim 1, the bolt beingmovable in a rearwards direction between a holding position in which theend part of the bolt projects from the housing and a non-holdingposition rearward of the holding position, the latch unit havingactuating means for moving the bolt in said rearwards direction from itsholding position to its non-holding position against said firstresilient bias means, the non-holding position being between the holdingposition and the retained position so that further movement of the boltin the rearwards direction to a position beyond the non-holding positionis required to attain the retained position.
 8. A latch unit accordingto claim 7 in which the second resilient bias means acts to resistmovement of the bolt between its non-holding and retained positions, butdoes not act to resist movement of the bolt between its holding andnon-holding positions.
 9. A latch unit according to claim 1characterized in that the retaining means is automatically activatedupon movement of the bolt into the retained position.
 10. A latch unitaccording to claim 1 characterised in that the retaining means isautomatically activated upon movement of the bolt into the retainedposition.
 11. A latch unit which includes a bolt, a housing for thebolt, guide means for the bolt in the housing, first resilient biasmeans for the bolt in the housing, the first resilient bias means urgingthe bolt in a forwards direction towards a holding position in which anend part of the bolt is outside the housing, retaining means forretaining the bolt in a retained position against movement in saidforwards direction, the retained position being spaced from the holdingposition, the retaining means comprising a peg and a slot, the retainingmeans having an active condition in which the peg is engaged with a partof the slot whereby to retain the bolt, second resilient bias means tohold the peg and slot in the active condition, the second resilient biasmeans acting to resist movement of the bolt to its retained position,wherein the peg is carried by the bolt and the slot is formed in amovable lever, and wherein a disengaging means can be operated to movethe lever whereby to overcome said second resilient bias means anddisengage the peg from said part of the slot so as to permit said endpart of the bolt to be urged towards the holding position by the firstresilient bias means.
 12. A latch unit according to claim 11characterized in that the disengaging means is an extension of thelever, said extension projecting from the housing.
 13. A latch unitaccording to claim 11 characterized in that the slot is a continuationof a second slot, the peg being movable along said second slot by saidfirst resilient bias means when said first part and said second part aredisengaged whereupon said end part is moveable into said holdingposition by the first resilient bias means.
 14. A latch unit accordingto claim 13 characterized in that the second slot has a ramp portion,and in that the first resilient bias means can act to hold said peg inengagement with said ramp portion.